Suppress warnings even in the imports

Some warnings were raised by `@asyncio.coroutine` decorators (e.g. in `asynctest` since Python 3.9.0), which are executed in class definition, which happens in the importing stage before pytest hooks are executed. By putting the warnings config to the INI file, we shift it to before the imports of any modules.

pytest.ini

@@ -0,0 +1,9 @@
+[pytest]
+# Unexpected warnings should fail the tests. Use `-W ignore` to explicitly disable it.
+# Warnings from the testing tools out of our control should not fail the tests.
+filterwarnings =
+    error
+    ignore:"@coroutine":DeprecationWarning:asynctest
+    ignore:The loop argument:DeprecationWarning:aiohttp
+    ignore:The loop argument:DeprecationWarning:aiojobs
+    ignore:The loop argument:DeprecationWarning:asyncio.queues

tests/conftest.py

@@ -27,15 +27,6 @@ def pytest_configure(config):
     config.addinivalue_line('markers', "e2e: end-to-end tests with real operators.")
     config.addinivalue_line('markers', "resource_clustered: (internal parameterizatiom mark).")
 
-    # Unexpected warnings should fail the tests. Use `-Wignore` to explicitly disable it.
-    config.addinivalue_line('filterwarnings', 'error')
-
-    # Warnings from the testing tools out of our control should not fail the tests.
-    config.addinivalue_line('filterwarnings', 'ignore:"@coroutine":DeprecationWarning:asynctest.mock')
-    config.addinivalue_line('filterwarnings', 'ignore:The loop argument:DeprecationWarning:aiohttp')
-    config.addinivalue_line('filterwarnings', 'ignore:The loop argument:DeprecationWarning:aiojobs')
-    config.addinivalue_line('filterwarnings', 'ignore:The loop argument:DeprecationWarning:asyncio.queues')  # aiojobs
-
 
 def pytest_addoption(parser):
     parser.addoption("--only-e2e", action="store_true", help="Execute end-to-end tests only.")